An aluminum product such as an automobile heat exchanger is welded through a brazing method by using fluoride-based flux, in an inert gas atmosphere. The brazing method is referred to as a Nocolok brazing method (a brazing method for an aluminum alloy using potassium fluoroaluminate flux), which is conducted by using: a brazing sheet obtained by cladding a filler material, such as an Al—Si-based JIS alloy 4045 (Al-10 mass % Si) or an Al—Si-based JIS alloy 4343 (Al-7.5 mass % Si), on one side or both sides of a core material of aluminum or an aluminum alloy, at a cladding ratio of 5 to 15%; or a brazing wire of the above-mentioned alloy. Then, the brazing sheet is formed or the brazing wire is provided at a site to be welded, to thereby assemble a product. Then, fluoride-based flux containing, as a main component, KAlF4, K2AlF5, K2AlF5.H2O, K3AlF6, or the like, is suspended in water or another solution in about 5 mass %, and the suspension is applied to an aluminum member. Then, the resultant is heated in a brazing furnace (at an oxygen concentration of 1,000 ppm or less and a dew point of −35° C. or lower) to a predetermined temperature in an atmosphere of an inert gas such as a nitrogen gas.
By containing magnesium, the resultant aluminum alloy has enhanced strength, to thereby realize a product of lighter in weight and thinner in thickness thereof. However, in a conventional Nocolok brazing method (a brazing method for an aluminum alloy using potassium fluoroaluminate flux), fluorine in flux and magnesium in an aluminum alloy react with each other, to form a high melting point compound such as magnesium fluoride (MgF2) on an aluminum surface. It is assumed that the reaction causes reduction to an effect of removing an oxide layer given by the flux, inhibits wetting and spreading of a filler material, and thereby degrades brazing property. Examples of a Nocolok brazing method for an aluminum alloy material containing magnesium include: a method, providing a magnesium diffusion barrier layer at an interface between a filler material of a brazing sheet and a core material containing magnesium, to suppress a reaction between flux and magnesium (see JP-A-6-63734, (“JP-A” meansunexamined published Japanese patent application) for example); and a method, incorporating cesium (Cs) into flux, to suppress formation of a high melting point compound such as magnesium fluoride (see JP-A-3-226396, for example). In the method involving providing a magnesium diffusion barrier, a barrier layer must have a thickness larger than a diffusion distance of magnesium in heat brazing, and then the brazing sheet itself cannot have a reduced thickness. Further, the flux containing cesium is very expensive, and thus the method involving incorporating cesium into flux has a disadvantage of insufficient mass productivity.